Cathode arm seal for fused salt electrolysis cells



M. R. FRANK 3,408,282

CATHODE ARM SEAL FOR FUSED SALT ELECTROLYSIS CELLS Oct. 29, 1968 Filed Jan. 11, 1965 FIG. i

FIG.3

INVENTOR MASON R` FRANK AGENT United States Patent O 3,408,282 CATHODE ARM SEAL FOR FUSED SALT ELECTROLYSIS CELLS Masen R. Frank, Niagara Falls, N.Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Jan. 11, 1965, Ser. No. 424,721 2 Clams. (Cl. 204-243) ABSTRACT OF THE DISCLOSURE A cathode arm seal is disclosed which is to be used in a "Down s type fused electrolysis cell. The seal is designed to allow a spacing between a circular metal plate, secured to the cell wall, and a seal ring, secured to the cathode arm, will accommodate cathode assembly to cell wall expansion, at Operating temperatures, and will prevent cell wall damage as a result of this expanson.

This invention relates to `fsed salt cells of the type having cathode arms or leads extending through the side walls of the cell. More particularly, the invention relates to flexible devices on the cathode arms of such cells which function both to prevent leakage of fused salt from the cell wall openings thorugh which the cathode arms extend and to prevent unequal displacement of the cathode with respect to the anode when the cell is heated to Operating conditions.

Fused salt cells such as the Downs cell for the electrolytc manufacture of sodium and chlorine from fused salt are commonly constructed with a steel cathode or assembly of cathodes having opposed arms or leads which extend through the side walls of the cell to the outside where they make contact with the electric power source. The cathode is generally circular in shape and is positioned concentrically around the anode. In operation, the level of .fused salt electrolyte in the cell is above the wall openings through which the cathode arms extend. This 'necessitates sealing those openings against leakage of the fused electrolyte therethrough. Sealing by filling the Wall openings with refractory cement has been commonly practiced but is far from satisfactory since expansion of the cathode upon heating the cell tends to cause movement of the cathode arms with resultant breakage of the seal and leakage of electrolyte.

Movement of the cathode arms upon heating to the Operating temperature, generally several hundred degrees centigrade above room temperature, results from the differential expansion of the cathode assembly with reference to the cell wall. Such differential expansion of the cathode not only tends to cause breakage of the seal between the cathode and the cell wall, but also often results in the cathode becoming displaced unequally with respect to the anode which is generally positioned within the cathode and concentrically with reference thereto. Such unequal displacement of the cathode results in an uneven annular spacing of the electrodes instead of the concentric spacing required for efiicient electrolysis.

It has been proposed to provide the cathode arms with flat plate shields in the form of a flange welded to the cathode arm. Such shields were intended to fit tightly against the innerface of the cell wall and thus at least aid in sealing the openings through which the cathode arms extend. However, such heat expansion of the cathode assembly often results in movement thereof largely in one direction. When this occurs one cathode arm will exert such excessive pressure against the cell wall as to cause distortion of the latter. As a result the seal is broken and leakage of electrolyte results. Furthermore, the concentric arrangement of the electrodes is disrupted. Because of such deficiencies; the above plate type seal or shield has been far from satisfactory.

Bergh et al. U.S. Patent 2,887,448 discloses a flexible cathode arm seal consisting of a dished flexible member or bellows welded about the cathode arm withthe outer edges of the member integrally joined to the cell wall around the wall opening so as to etfect a leak-proof seal. That type of seal has not been practical because of its relatively complicated construction and its requirement of st-ops welded internally onto the cathode arm or provided externally of the cell for preventing unequal displacement of the cathode with respect to the anode when the cell is heated.

It is an object of my invention to provide an improved cathode arm seal for use in fused salt cells of the type indicated above. A particular object is to provide a flexible seal of simple design and construction which includes a built-in stop whereby the device not only effectively seals the cell Wall opening against leakage but also effectively prevents unequal dsplacement of the cathode with respect ot the anode during heating of the cell. Still further objects will be apparent from the following description.

The cathode arm seal of my invention includes a circular metal plate welded about the cathode arm. The circular plate is rolled slightly at its outer edge to form a shallow dish facing the cell wall and is welded about its outer edge to a seal ring which fits tightly in sealing fashion against the inner surface of the cell wall around the opening therein through which the cathode arm eX- tends. The seal is so designed that the spacing between the circular metal plate and the seal ring is approximately one-half the differential expansion of the cathode assembly with respect to the cell wall that will occur when the cell is heated to Operating temperature.

The invention will now be described in greater detail in conjunction with the accompanying drawings in which:

FIG. 1 is a vertical sectonal View of a fused salt cell with the upper and lower parts thereof omitted but ircluding a cathode with opposing cathode arms extending through the cell walls and with each cathode arm provided with a exible seal constructed in accordance with the invention;

FIG. 2 is a horizontal sectonal view at A-A of FIG. 1; and

FIG. 3 is a somewhat enlarged fragmental vertical sectonal View showing more clearly the relationship between the cathode arm, the flexible seal and the cell wall.

In the drawings, numeral 1 represents a steel shell which encloses the cell. Shell 1 is provided with a refractory lining 2, generally constructed of refractory bricks. The circular metal, e.g., steel, cathode 3 is disposed within the cell concentrically around cylindrical anode 4, thus providing an annular space between the electrodes in which is positioned screen diaphragm 5. Cathode 3 is supported by opposing cathode arms 6 which extend through the cell wall and rest upon cell lining 2. I-n operation, the outer ends of cathode arms 6 are, of course, connected to an electric power source not indicated in the drawing.

Each cathode arm 6 is provided with a flexible seal device 7 consisting of a flexible circular plate 8 and a seal ring 10, each generally constructed of steel. circular plate 8 is tightly sealed, e.g., by Weldng, around cathode arm 6 in the form of a wide flange whose outer edge 9 is rolled slightly and is tightly sealed, e.g., by welding, to the face of seal ring 10. The latter fits tightly in sealing fashion against the inner surface of cell lining 2 around the cathode arm. In' order that the fit between seal ring 10 and cell lining 2 :be leak-proof, it is generally advisable to mortar seal ring 10 to lining 2 using a refractory cement.

It is essential that seal 7 be constructed so that flexible circular .plate 8 and seal rinbe spaced apart from each other except where edge 9 of plate 8 is joined to ring 10.`

Such spacing is conveniently accomplished by rolling edge 9 outwardly so as to make plate 8 slightly dish shaped. The space between flexible plate 8 and seal ring 10 permits movement of cathode arm 6 towards the cell wall due to expansion of the cathode assembly (i.e., cathode 3 and cathode arm 6) when the cell is heated to Operating temperature. During such movement, cathode arm 6 will slide on supporting cell lining 2 and cause plate 8 to fiex towards seal ring 10. By making leak-proof all of the joints between plate 8 and cathode arm 6, between edge 9 of plate 8 and seal ring 10, and between seal ring 10 and wall lining 2, such movement of cathode arm 6 during heating will occur without leakage of the fused electrolyte from the openings in the cell wall through which the cathode arms extend.

The width of the space between the body portions of flexible plate 8 and seal ring 10 isgoverned -by the extent to which edge 9 of plate 10 is rolled outwardly. The width of that space is critical and should be substantially equal to one-half the differential expansion of the cathode assembly with respect to the cell wall. Such difi erential expansion can -be readily calculated from the known coeicients of expansions for the materials involved and the temperature to which the cell is to -be heated. Thus, if the differential expansion is inch, the space between plate 8 and ring 10 of the flexible seal 7 on each cathode arm should be 7 inch. By constructing flexible seals 7 so that the space in each seal between plate 8 and ring 10 is '7 inch, movement of the cathode assembly in a single direction is limited to inch. This is so, because when a cathode arm has moved that distance toward the side wall, flexible plate 8 of the seal will have been flexed so as to contact ring 10. Such contact stops further movement of the cathode assembly in that direction. Thus, flexible seal 7, by its indicated Construction, will include a "built-in stop which limits movement of the cathode arm towards the cell wall to a distance not more than one-half the differential expansion of the cathode assembly with respect to the cell wall. By providing each opposing cathode arm with such a flexible seal, the expansive movement of the cathode assembly resulting from the heating of the cell to the Operating temperature will be distributed evenly and in opposite directions to the two cathode arms. The net result is that the concentric spacing of cathode 3 from anode 4 will not be disturbed as a result of the heating of the cell.

In order that the "built-in" stop of the flexible seal function in the manner intended, it is important that the space between flexible plate 8 and seal ring 10 be kept void during Construction of the seal. Thus, care should be exercised to keep mortar out of the above space when mortaring seal ring 10 to wall lining 2. This can be readily and conveniently accomplished by shielding that space with a plastic film such as cellophane sheet material or the like at least during the mortaring operation.

Circular plate 8 of seal 7 should be sufficiently flexible to permit or accomodate the expansive movement of the cathode arm caused by the heating of the cell. Its flexibility will be determined largely by its thickness and diameter. A steel plate about inch thick and 20 inches in diameter will possess the required flexibility and will be sufiiciently rugged to give excellent results. Seal ring 10 should of course be as large in outer diameter as plate 8 to which it is welded. This ring should be relatively wide, e.g., on the order of at least 2 inches, preferably wider, so as to provide a large surface for scaling with the cell lining and also to distribute the thrust resulting from the movement of the cathode over a large area of the cell wllfEven distribution of such thrust over the wall area contacted requires that ring 10 be of substantial thickness. Steel rings about 18 to 24 inches in outside diameter, 2 to 3 or more inches wide and M: inch thick give excellent results. In general, the ring should be dimensioned to provide as large an area for contact with the cell wall as the structure of the cell will permit.

The flexible cathode arm seals of my invention have been found in actual use in a "Downs type cell for the electrolytic production of sodium'and chlorine from fused salt to be highly efifective in scaling the cell against leakage of electrolyte and in preventing unequal displacement of the cathode with respect to the anode when the cell is heated to Operating temperature. Because of their high eifectiveness in the above respects and their simple construction, the seals are admirably suited for .use in combination with cathode arms in any fused salt cell having opposing cathode arms extending through the cell walls at a level below the level of the used salt electrolyte in the cell. Thus, they are usa-ble in cells having a single cathodeanode unit, as illustrated in the drawings, and also in cells having a plurality of such units in the form of an assembly of circular cathodes (each having an anode positioned therein) having opposing cathode arms which extend through the cell walls. A cell having such an assembly of four cathodes is shown in Gallnger U.S. Patent 2,824,558.

The embodiments of my invention in which an exclusive property or privilege is claimed are as follows:

1. In a fused salt electrolysis cell comprising a cell wall including an outer metal shell and an inner refractory lining and having a cathode assembly therein including a circular cathode with opposed cathode arms extending laterally through the cell wall at a level below the level of the fused salt in the cell, said cathode being positioned concentrcally about an anode, the combnation with each of said cathode arms of a seal device comprising a flexible circular metal plate tightly sealed about the cathode arm, said plate being dished outwardly and sealed tightly at its outer edge to the face of a seal ring fitted tightly against only the inner surface of said refractory lining of the cell wall around said cathode arm and being non-integral with said metal shell, said circular plate and said seal ring being spaced from each other at a predetermined distance which controls by mutual contact at the inner surface of said seal ring the extent of the cxpansive movement of the cathode arm towards the cell wall when the cell is heated to operat ing temperature, said distance being substantially the same in the seal device for each cathode arm whereby unequal displacement of the cathode with respect to the anode is prevented during said heating of the cell.

2. The combnation according to claim 1 wherein the predetermined distance between the flexible circular metal plate and the seal ring of the seal device for each cathode arm is one-half the dififerential expansion of the cathode assembly with respect to the cell wall when the cell is heated to Operating temperature.

References Citel UNITED STATES PATENTS 2,621,155 12/1952 Williams. 2,887,448 5/ 1959 Bergh et al.

JOHN H. MACK, Pr'mary Exam'ner.

D. R. VALENTINE, Assistant Exam'ner. 

